Low pressure mercury plasma discharge tube



Aug. 6, 1963 K. WEHNER LOW PRESSURE MERCURY PLASMA DISCHARGE TUBE Filed April 14, 1961 MERCURY DIFFUSION PUMP INVENTOR. GOTTFRIED K. WEHNER zg k TO MERCURY DIFFUSION PUMP ATTORNEY United States Patent 3,166,272 LOW PRESSURE MERQURY PLASSMA DISCHARGE TUBE Gottfried K. Wehner, Minneapolis, Minn, assignor to General Mills, inc a corporation of Delaware Filed Apr. 14, 1961, Ser. No. 103,656 12 (Jiaims. (Cl. 3l3--7) The present invention relates to improvements in vacuum arc discharge tubes. More particularly, it pertains to low pressure mercury plasma discharge tubes.

One object of the present invention is to provide a low pressure mercury plasma tube in which the chamber or envelope member enclosing the anode section thereof can be removed without disturbing the electrical connections to the electrodes positioned in said anode section.

A further object of the present invention is to provide a low pressure mercury plasma tube in which the passage between the cathode section thereof and a mercury diffusion pump has a large opening of short length in order to achieve high pumping speeds and cleaner discharge conditions within said cathode section.

A yet further object of the present invention is to provide a low pressure plasma tube for establishing plasmas other than mercury plasmas, e.g., plasmas of noble gases.

Other objects and advantages in the invention will be apparent from the following description in which certain preferred embodiments of the invention are disclosed.

In the drawings which form a part of this application:

FIGURE 1 is a perspective view of a low pressure mercury plasma tube according to the present invention; and

FIG. 2 is a cross-sectional view of the tube shown in FIG. 1; and

FIG. -3 is a circuit diagram that may be used to operate the tube of FIG. 1.

Referring now to the figures, FIGURE 1 depicts a low pressure mercury plasma tube 1%} having an upper chamber or envelope section 12 and a lower section or tank assembly 14. The tank assembly 14 has a conduit 15 connected thereto.

As shown in FIGURE 2 the conduit 15 connects the interior of the tank assembly 14 to a mercury diffusion pump (not shown). The tank assembly 1 2 includes a first cylindrical wall 16, a second cylindrical wall 18, and a third cylindrical wall 20 located between and spaced from said first and second walls to storm reservoirs 22- and 24 between said walls. The wall 18 forms a short length, large diameter passage 19 which connects with one end of the conduit 15. The reservoir 22 formed between the walls 16 and 20 is adapted to contain water while the reservoir 24 formed between the walls 20 and 18 is adapted to contain a mercury pool 26. A cathode spot anchor 27 is positioned in the mercury reservoir. The anchor 27 is a strip of molybdenum formed in a circle which has a diameter equal to the distance from the outside surface of the wall 16 to the inside surface of the wall 20. The strip of molybdenum can be in any form as long as it surrounds the point at which an igniter 52 comes in contact with the surface of the mercury pool 26.

The wall 20 has on its upper end a shoulder member comprising a shoulder portion 28, a radially extending flange portion 30, an upstanding wall portion 32', and an upstanding collar portion 34. The shoulder portion 28 is provided with a passage 35. The passage 35 provides means whereby gas can be admitted into the interior of the envelope =12. The upstanding wall portion 32 has an annular recess 36 therein. The recess 36 contains rubber O-ring seal 38. The envelope 12 is 3,l,272 Patented Aug. 6, 1Q63 firmly and releasably positioned in the recess 36 on the O-ring seal 38. This connection between the tank assembly 14 and the envelope 12 provides a gas tight seal therebetween.

The envelope 12 can be a Pyrex glass bell jar. It is also possible to make the envelope 12 of other materials, such as metal. The envelope 12 has a re-entrant cold trap cavity 39 formed therein which extends into the interior of said envelope 12. This cavity 39 is constructed so that it can receive cool materials such as liquid nitrogen. When filled with such cool materials, mercury ions in the envelope 12 stick to the surface of the cavity 8-9 when they come in contact therewith. Said surface is disposed in the interior of the envelope 12.

The interior of the envelope 12 is separated from the interior of the tank assembly 14 by a circular top plate 40 having a centrally located hole 42. The plate 40 is disposed in a seat 4 formed in the shoulder portion 28. As shown the plate 49* is insulated from the shoulder portion 28.

The wall 13 extends upwardly toward said plate 40 so as to provide a passage way 46 between the mercury pool reservoir 24 and the passage 19 formed by said wall =18. The passage 19 has a large opening of short length which is adapted to connect the reservoir 24 to a mercury diffusion pump (not shown). The passage 19' is concentrically surrounded by the mercury pool reservoir 24.

A plurality of electrodes are supportedly connected to the tank assembly 14. The plurality of electrodes include a fine mesh graphite grid Sil positioned in the top plate hole 42, an igniter 52 adapted to be in contact with the mercury pool 26, an auxiliary anode 54 concentrically located with regard to the passage 19 between the mercury pool 26 and the grid 50, :an anode 56 disposed in the envelope 12, and an electrode 58 adapted to hold a target in the envelope 12.

The grid 50 separates the tube 10 anode space from the cathode space thereof. The grid 50 may be about 0.4 mm. thick having six holes per cm. of length, said holes being 1.3 mm. diameter as explained in pages 690- 704 of vol. 1 02, No. 3 of The Physical Review. The grid 50 helps to stabilize the main discharge in the tube 16 and permits a considerable increase in plasma density within the anode space in the upper envelope 12 without utilization of undesirably high discharge currents and also permits an appreciable, yet simple, control of the velocity of accelerated beam electrons by variations on the grid potential.

Individual insulated covered electrical leads 60, 62, 64, 66, and 68 extend from the grid St), the igniter 52, the auxiliary anode 54, the anode 56, and the target electrode 58, respectively, to the outside of the tube 10 through the tank assembly 14. Electric lead 69 is connected to the tank assembly 14. As indicated the electrical leads 60, 62, 64, 66, and 68 are insulated from the tank assembly by appropriate means such as glass insulators 7t), 72, 74, 76, and 73, respectively. These insulators are held in place by appropriate means such as screws and O-rings as is well known in the art in such a way that they are gas tight.

The electric leads 60, 62, 64, 66, 68, and 69 are connected to a ground as shown in FIGURE 3. The electric lead 60 is connected to the ground 80 through a DC. power supply 82 and a resistor 84 connected in series. The electric lead 69 which is electrically linked through the stainless steel tank 14 to the molybdenum strip 27 is connected directly to the ground 80. The electric lead 62 is connected to one terminal of a two way switch 86. A second terminal of the two way switch '86 is connected to ground through a DC. power supply 88 while a third terminal of the switch 86 is also connected to ground through a condenser 98. The switch 86 is normally in the position indicated by the solid line. When it is desired to activate the igniter 52 the switch 86 is momentarily moved to its dotted line position. The lead 64 is connected through a resistor 92 to the power supply 88. The common junction 94 between the resistor -92 and the power supply 88 is connected to one end of a variable resistor 96. The other end of the variable resistor 96 is connected to the lead 66. The lead 68 is linked through a DC. power supply 98 and a resistor 10!) connected in series to a junction 102 between resistor 96 and lead 66.

In a typical operation the herein described low pres sure mercury plasma discharge tube functions substantially as follows with the mercury pool 2'6, the cathode spot anchor 27, the igniter 52, and the auxiliary anode 54 serving as the cathode section for the discharge to the main anode 56 in the envelope 12. A target, i.e., a material which one desires to subject to mercury ionic bombardment, is connected to the electrode 58. The envelope 12 is then .placed in position as shown in FIG- UR=E 2. The mercury difiusion pump then lowers the pressure within the tank assembly 14 and envelope 12 to about 1 micron. By having the passage 19 between the tank assembly 14 and the pump constructed to provide a short length large opening therebetween as taught herein one can achieve higher pumping speeds and cleaner discharge conditions within the tube. Then a DC. discharge of approximately 3 amps. at 15 volts voltage drop is ignited between the mercury pool 26 and the auxiliary anode 54 by means of a current pulse to the igniter 52.. The current pulse is obtained by momentarily moving the switch 86 from its solid line position to its dotted line position. Since the main anode 56 is connected through resistor 96 to the DC. power supply 88, the main discharge of approximately 5 amps. at 30 volts voltage drop between the pool 26 and the anode '56 can be established. This causes the anode section of the tube, i.e., the space inside the envelope 12 above the grid 5%), to become filled with a low pressure mercury plasma with a density of the order of to 10 ions/emit The target attached to the electrode 58 becomes surrounded by this plasma. When said target is negative with respect to the anode or the plasma it is subject to ion bombardment.

When it is desired to change the target, the tube and pump are shut off and the envelope is removed. It will be noted that with the tube constructed in accordance with the teachings herein set forth that the envelope can be easily and rapidly removed without disturbing any of the electric leads 60, 62, 64, '66, 68 and/or 69. Thus, I have taught how to construct a low pressure mercury plasma discharge tube which can be quickly and conveniently opened for removing and inserting targets.

As is well known in the art the water temperature between the walls 16 and 20 can be used to control the mercury vapor pressure in the tube (at C.; 1 micron).

When it is desired to work in a glass plasma other than mercury e.g., a plasma of a noble gas like He, Ne, A Kr and Xe, one can freeze out the mercury ions from the anode portion of the tube, i.e., the portion of the tube enclosed by the envelope 12 above the grid 50, by introducing liquid nitrogen into the re-entrant cold trap cavity 39 of the envelope 12 and then admit the gas which is desired through the gas inlet passage 35. In this way one can operate a gas discharge other than mercury in the main anode portion of the tube, but retain the very convenient mercury pool type cathode which is an eificient and inexhaustable source for electrons ready for operation immediately after pump down. In both mercury and noble gas discharges, the background pressure of reactive gases is down to the 10* to 10- mm. mercury range.

In view of the principles set forth herein, I have shown some of the ways of carrying out the present invention and some of the equivalents which are suggested by these disclosures.

Now, therefore, I claim:

1. A low pressure plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir being adapted to be connected to a vacuum pump, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope, and individual electric leads extending from said electrodes to the outside of said tube through said tank assembly so said envelope can be separated from said assembly without disturbing said leads.

2. A low pressure mercury plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a short length large opening passage adapted to connect said reservoir to a vacuum pump, said envelope being firrmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope and individual electric leads extending from said electrodes to the outside of said tube.

3. A low pressure mercury plasma tube as set forth in claim 2 in which said mercury pool reservoir concentrical-1y surrounds said short length large opening passage.

4. A low pressure mercury plasma tube comprising a tank assembly, an envelope and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a short length large opening passage adapted to connect said reservoir to a vacuum pump, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope and individual electric leads extending from said electrodes to the outside of said tube through said tank assembly so said envelope can be separated from said assembly without disturbing said leads.

5. A low pressure mercury plasma tube as set forth in claim 4 in which said mercury pool reservoir concentrical ly surrounds said short length large opening passage.

6. A low pressure mercury plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a passage adapted to connect said reservoir to a vacuum pump, a re-entr-ant cold trap adapted to extend into the interior of said envelope, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope and individual electric leads extending from said electrodes to the outside of said tube.

7. A low pressure mercury plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a passage adapted to connect said reservoir to a vacuum pump, a re-entrant cold trap adapted to extend into the interior of said envelope, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope, and individual electric leads extending from said electrodes to the outside of said tube through said tank assembly so said envelope can be separated from said assembly without disturbing said leads.

8. -A low pressure mercury plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a short length large opening passage adapted to connect said reservoir to a vacuum pump, a reentran-t cold trap adapted to extend into the interior of said envelope, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope, and individual electric leads extending from said electrodes to the outside of said tube.

9. A low pressure mercury plasma tube comprising a tank assembly, an envelope, and a plurality of electrodes, said tank assembly including a mercury pool reservoir, said reservoir having a short length large opening passage adapted to connect said reservoir to a vacuum pump, a reentrant cold trap adapted to extend into the interior of said envelope, said envelope being firmly and releasably positioned on said tank assembly, said plurality of electrodes including an anode, a target electrode, a grid, an auxiliary anode, and an igniter, said anode and target electrodes being positioned in said envelope, and individual electric leads extending from said electrodes to the outside of said tube through said tank assembly so said envelope can be separated :firom said assembly without disturbing said leads.

10. A low pressure mercury plasma tube comprising a tank assembly, an upper envelope, and a plurality of electrodes, said tank assembly including an outer wall and an inner wall which forms a passage adapted to be connected to a vacuum pump, said inner Wall being spaced from said outer Wall to form a reservoir between said walls, said reservoir being adapted to contain a mercury pool, said inner wall providing a passage way between said mercury pool reservoir and said inner wall passage, said outer wall having on its upper end a shoulder member, said shoulder member having an opening, said envelope Ibeing firmly and releasably seated on said shoulder member, said plurality of electrodes including a grid disposed in said shoulder opening, an igniter adapted to be in contact :with said mercury pool, an auxiliary anode positioned between said mercury pool and said grid, an anode in said envelope, said anode being supportedly connected to said tank assembly, an electrode adapted to hold a target in said envelope, said target electrode supportedly connected to said tank assembly, and individual electric leads extending from said anode, said target electrode, said grid, said auxiliary anode and said igniter to the outside of said tube through said tank assembly whereby said envelope can be separated from said tank assembly Without disturbing said leads.

11. A low pressure mercury plasma tube as set forth in claim 10 in which said reservoir concentrically surrounds said inner wall passage.

12. A low pressure mercury plasma tube as set forth in claim 10 which has a re-entrant cold trap for-med by a cavity in said envelope that extends into the interior of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A LOW PRESSURE PLASMA TUBE COMPRISING A TANK ASSEMBLY, AN ENVELOPE, AND A PLURALITY OF ELECTODES, SAID TANK ASSEMBLY INCLUDING A MERCURY POOL RESERVOIR, SAID RESERVOIR BEING ADAPTED TO BE CONNECTED TO A VACUUM PUMP, SAID ENVELOPE EING FIRMLY AND RELEASABLY POSITIONED ON SAID TANK ASSEMBLY, SAID PLURALITY OF ELECTRODES INCLUDING AN ANODE, A TARGET ELECTRODE, A GRID, AN AUXILARLY ANODE, AND AN IGNITER, SAID ANODE AND TARGET ELECTRODES BEING POSITIONED IN SAID ENVELOPE, AND INDIVUAL ELECTRIC LEADS EXTENDING FROM SAID ELECTRODES TO THE OUTSIDE OF SAID TUBE THROUGH SAID TANK ASSEMBLY SO SAID ENVELOPE CAN BE SEPARATED FROM SAID ASSEMBLY WITHOUT DISTRIBUTING SAID LEADS 